Authorities in Britain and France have been trying to trace the carcasses of six horses contaminated with bute, which were slaughtered in a UK abattoir and may have entered the human food chain on mainland Europe.
Why is the substance considered such a risk? Here's what you need to know.
What is bute?
It is a veterinary painkiller. Its full name is phenylbutazone, and it is used by vets on horses to treat inflammations.
What does the law say about bute?
It is banned from the human food chain. No horse that has ever received the drug, no matter how long ago, is meant to be slaughtered for meat for human consumption.
Is bute dangerous to humans?
Yes. It can cause a potentially fatal blood disorder called aplastic anaemia, in which the bone marrow fails to produce enough blood cells.
But are we at risk?
The Food Standards Agency advice has been that the risk exists but is very low. Aplastic anaemia is rare, and evidence suggests around one person in 30,000 exposed to bute when used as a human medicine suffered some kind of serious side-effect. However these people were exposed to significantly higher doses than anything likely to have entered the food chain.
Why is there uncertainty about the danger?
It's not possible to say what triggers the disorder, and therefore it's not possible to identify a safe level of residue in meat.
The UK's chief medical officer Dame Sally Davies has said that although the drug has been linked to side-effects in patients who have been taking it as a medicine for arthritis, the risk remains very low.
"If you ate 100% horse burgers of 250g, you would have to eat, in one day, more than 500 or 600 to get to a human dose," she said.
What has caused the current scare?
Bute was detected in eight horses out of 206 tested by the Food Standards Agency (FSA) in the first week of this month.
Six of the horses had been slaughtered by LJ Potter Partners at Stillman's (Somerset) Ltd in Taunton, Somerset.
The other two were killed at High Peak Meat Exports Ltd in Nantwich, Cheshire. These did not leave the slaughterhouse and have since been destroyed. But the six from Somerset were sent to France, where horse meat is commonly eaten.
Are horse carcasses regularly tested for bute?
No. FSA chief executive Catherine Brown has explained that the agency increased testing of horse carcasses over a three-month period last year after intelligence from abattoirs suggested bute was getting into the food chain.
Of 63 tested - amounting to 5% of all carcasses - four (6%) tested positive for the painkiller, prompting the FSA to start testing 100% of horse meat in January, which revealed the eight contaminated carcasses.
Authorities now suspect that bute has been entering the food chain for some time.
What safeguards are in place to prevent bute entering the food chain?
Both vets and horse owners have to sign horse passports if an animal is treated with bute, to ensure it is not subsequently sold on for human consumption.
"If both these people have done the right thing, horses with bute in don't make their way into the food chain," says Catherine Brown. "Someone has always broken the rules."
What have the abattoirs involved in the scare said?
In a statement, LJ Potter Partners said it had taken steps to recall meat delivered to France and had warned the government that horse passport regulations would not ensure public health:
"We wish to produce meat that is wholesome, nutritious, good value and, most importantly, safe.
"The current EU regulations have been proved, beyond all doubt, to be ineffective in permitting us to do this; they additionally are inappropriately excluding large numbers of horses from the legitimate food chain.
"It is our belief that this has been the causal condition that has led to large numbers of horses entering the food chain illegitimately.
"The fraudulent misrepresentation of horse meat is a direct consequence of ignorant, mis-informed, badly drafted EU legislation."
What has the government said?
The prime minister's official spokesman has said: "Bute should not be present in horses that go into the food chain. It is incredibly important that we get to the bottom of what is happening. We are working very closely with French authorities on tracing the carcasses involved that went to France. We are doing that as a matter of urgency with the French authorities."
And what is happening in Europe?
At an emergency summit in Brussels on Thursday night, EU ministers agreed to the random testing of meat products across Europe for both horse DNA and bute.